(a) Field of the Invention
This invention relates to a method for soldering a lead terminal of an electronic component to a terminal connecting portion on a printed substrate with a paste-like solder and, more particularly, to a soldering method adapted to connect an electronic component having a J-bend type or Butt-lead type lead terminal to a terminal connecting portion on a printed substrate with a paste-like solder.
(b) Description of Prior Art
A conventional method for soldering a lead terminal of an electronic component having a J type lead terminal on a terminal connecting portion of a printed substrate has, as shown in FIG. 1, the steps of placing paste-like solders 3, 3 on conductive pads 2, 2 formed as terminal connecting portions on a printed substrate 1, holding the lead terminals 5, 5 of an electronic component 4, such as an IC pack at the paste-like solders 3, 3 by means of the viscosity of the paste-like solders 3, 3, heating the substrate while moving the entire substrate, for example, by a belt conveyor (not shown) in a heating atmosphere furnace (not shown) to melt the paste-like solders 3, 3 to allow the melted paste-like solders 3, 3 to flow from the pads 2, 2 to lead terminals 5, 5 by means of a capillary phenomenon to connect them. Thus, the method for heating the entire substrate, for example, by moving the entire substrate by the belt conveyor in the heating atmosphere furnace is known as a vapor phase reflowing method or an infrared ray method.
In this conventional soldering method, the temperature rising rate of the pads 2 and the lead terminals 5 on the printed substrate 1 moving in the heating atmosphere furnace are slightly different due to the difference of the thermal capacities of the materials of the printed substrate 1 and the lead terminal of the electronic component 4, and the latter is, as shown in FIG. 3, generally larger than the former. This is because the substrate material is made of synthetic resin, such as epoxy resin while the material of the lead terminal 5 is made of solder-plated copper and solder-plated iron-nickel alloy. Thus, since the temperature of the pad 2 formed on the printed substrate 1 is very low when the lead terminal 5 of the electronic component 4 arrives at the melting point of the paste-like solder 3, part of the paste-like solder 3 contacted with the lead terminal 5 is melted earlier than the portion of the paste-like solder contacted with the pad, and most of the melted solder is bonded, as shown in FIG. 2, in the state attracted to the lead terminal 5 to cause it to be improperly connected as its drawback. In order to reduce the difference of the above-mentioned temperature rising rates, it is tried to preheat the substrate, but it is difficult to suitably control the temperature in response to the external conditions, such as atmospheric temperature and moisture, a facility therefore is required, and the substrate might be damaged. Thus, a soldering method which has high efficiency of securing a number of electronic components at predetermined positions on a substrate without damaging the substrate is desired.